Mars: February 2008

Throughout recorded Earth history, microbial life has been ubiquitous, abundant, metabolically diverse, and, for the Precambrian four billion years of geologic time, biotically predominant. In the search for life elsewhere in the Cosmos, the prokaryote-dominated world of the Precambrian is the best analogue we know. Though evidence of microbe-level life will thus be sought in rocks returned from Mars, for the first such missions only minimal samples will be available. This raises important questions: What amount of rock is needed to detect past life? What evidence is required to establish biogenicity? How can true fossils be distinguished from contaminants?